# std::experimental::optional<T>::optional

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< cpp‎ | experimental‎ | optional

constexpr optional() noexcept; constexpr optional( std::experimental::nullopt_t ) noexcept; | (1) | (library fundamentals TS)
optional( const optional& other ); | (2) | (library fundamentals TS)
optional( optional&& other ) noexcept(/* see below */); | (3) | (library fundamentals TS)
constexpr optional( const T& value ); | (4) | (library fundamentals TS)
constexpr optional( T&& value ); | (5) | (library fundamentals TS)
template< class... Args > constexpr explicit optional( std::experimental::in_place_t, Args&&... args ); | (6) | (library fundamentals TS)
template< class U, class... Args > constexpr explicit optional( std::experimental::in_place_t, std::initializer_list<U> ilist, Args&&... args ); | (7) | (library fundamentals TS)

Constructs a new `optional` object.

1) Constructs the object that does not contain a value.
2) Copy constructor: If other contains a value, initializes the contained value as if direct-initializing (but not direct-list-initializing) an object of type `T` with the expression *other. If other does not contain a value, constructs an object that does not contain a value.
3) Move constructor: If other contains a value, initializes the contained value as if direct-initializing (but not direct-list-initializing) an object of type `T` with the expression std::move(*other) and does not make other empty: a moved-from optional still contains a value, but the value itself is moved from. If other does not contain a value, constructs an object that does not contain a value.
4) Constructs an optional object that contains a value, initialized as if direct-initializing (but not direct-list-initializing) an object of type `T` with the expression value. This constructor is `constexpr` if the constructor of `T` selected by direct-initialization is `constexpr`.
5) Constructs an optional object that contains a value, initialized as if direct-initializing (but not direct-list-initializing) an object of type `T` with the expression std::move(value). This constructor is `constexpr` if the constructor of `T` selected by direct-initialization is `constexpr`.
6) Constructs an optional object that contains a value, initialized as if direct-initializing (but not direct-list-initializing) an object of type `T` from the arguments std::forward<Args>(args)....
7) Constructs an optional object that contains a value, initialized as if direct-initializing (but not direct-list-initializing) an object of type `T` from the arguments ilist, std::forward<Args>(args).... The function does not participate in the overload resolution if std::is_constructible<T, std::initializer_list<U>&, Args&&...>::value != true.

### Parameters

other | - | another `optional` object whose contained value to copy
value | - | value to initialize the contained value with
args... | - | arguments to initialize the contained value with
ilist | - | initializer list to initialize the contained value with
Type requirements
- `T` must meet the requirements of CopyConstructible in order to use overloads (2,4).
- `T` must meet the requirements of MoveConstructible in order to use overloads (3,5).

### Exceptions

2) Throws any exception thrown by the constructor of `T`.
3) Throws any exception thrown by the constructor of `T`. Has the following `noexcept` declaration:
`noexcept` specification:
noexcept(std::is_nothrow_move_constructible<T>::value)
4-7) Throws any exception thrown by the constructor of `T`.

### Example

Run this code

    #include <experimental/optional>
    #include <iostream>
    #include <string>

    int main()
    {
        std::experimental::optional<int> o1,      // empty
                                         o2 = 1,  // init from rvalue
                                         o3 = o2; // copy-constructor

        std::experimental::optional<std::string> o4(std::experimental::in_place,
                                                    {'a', 'b', 'c'});

        std::cout << *o2 << ' ' << *o3 << ' ' << *o4 << '\n';
    }

Output:

    1 1 abc

### See also

make_optional | creates an `optional` object (function template)

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